1. Field of the Invention
The present invention relates to a multilayer optical recording medium having a plurality of interfacial surfaces that reflect an incident light beam.
2. Description of the Related Art
Recently, optical recording media, such as CDs (compact discs), DVDs (digital versatile discs), and BDs (Blu-ray discs (trade name)), have been in widespread use.
Among such recording media, recordable optical recording media having a recording film (a recording layer) have a plurality of recording layers in order to increase the recording capacity. In particular, in recent years, multilayer optical recording media having three or more recording films have been developed (refer to, for example, Japanese Unexamined Patent Application Publication No. 2004-213720).
However, in multilayer optical recording media, so-called interlayer stray light is generated. Thus, a problem of multiple interference arises. As used herein, the term “multiple interference” refers to a phenomenon in which a reflected light beam from a recording film from which information is to be reproduced is interfered with light beams reflected from three or more interfacial surfaces (light reflective interfacial surfaces) other than the recording film for reproduction and, thus, a variation in light intensity occurs on a detector.
Naturally, it is desirable to prevent the occurrence of multiple interference in order to prevent a decrease in the reproduction performance.
For triple-layer optical recording media having three recording films, the occurrence of multiple interference can be prevented by making the thicknesses of spacers formed between the recording films different from one another.
This can be understood by looking at the case in which a triple-layer optical recording medium has a spacer between recording films Lrc0 and Lrc1 (Lrc0 indicates the lowermost recording film) with a thickness that is the same as the thickness of a spacer between the recording film Lrc1 and a recording film Lrc2, and information in the recording film Lrc0 is reproduced. That is, in this case, the optical path length of a light beam focused onto the recording film Lrc0 and reflected by the recording film Lrc0 (a reproduction light beam—an interfered light beam) is the same as the optical path length of a light beam sequentially reflected by the recording film Lrc1, the recording film Lrc2 (the lower surface), and the recording film Lrc1 (i.e., a stray light beam). Thus, these light beams are interfered with one another and, therefore, multiple interference occurs.
Accordingly, for a triple-layer optical recording medium, by employing different thicknesses for the spacers formed between the recording films, the optical path length of the interfered light beam can be made different from the optical path length of the stray light beam. In this way, the occurrence of the multiple interference can be prevented.
However, for a multilayer optical recording medium having four or more layers, it is difficult to prevent the occurrence of multiple interference only by employing different thicknesses for the spacers.
For example, as shown in FIG. 17, a multilayer optical recording medium having five recording films Lrc (i.e., recording films Lrc0 to Lrc4) is discussed. A spacer S1 having a thickness of 4 is formed between the recording films Lrc0 and Lrc1. A spacer S2 having a thickness of 2 is formed between the recording films Lrc1 and Lrc2. A spacer S3 having a thickness of 3 is formed between the recording films Lrc2 and Lrc3. A spacer S4 having a thickness of 1 is formed between the recording films Lrc3 and Lrc4. In this way, the thicknesses of the spacers are set to different values.
In such a case, when information recorded in the lowermost recording film Lrc0 is reproduced, the optical path length of a stray light beam reflected by the recording films Lrc2, Lrc4, and Lrc1 in this order (indicated as a thin solid arrow in FIG. 17) is the same as the optical path length of a stray light beam reflected by the recording films Lrc1, Lrc4, and Lrc2 in this order (indicated as a thin dotted arrow in FIG. 17) for a reproduction light beam (indicated as a thick solid arrow in FIG. 17). Thus, the two stray light beams interfere with the reproduction light beam. This is because the thickness of the spacer S1 is the same as the sum of thicknesses of the spacers S3 and S4.
As described above, in a multilayer optical recording medium having four or more recording films, it is difficult to prevent the occurrence of the multiple interference only by employing different thicknesses for the spacers.